1. Field of the Invention
The present invention relates to an image forming apparatus which forms an image by an electrophotographic method such as a copying machine and a printer, in particular to an image forming apparatus having a cooling portion for preventing a temperature rise in the apparatus.
2. Description of the Related Art
When an image forming apparatus operates continuously, a temperature of parts of the apparatus rises. Parts of which temperature rises include a developing device where a friction heat is easily produced by toner agitation, a fixing device for fixing toner to a sheet, a power supply, a motor unit, and the like.
Currently, a toner having thermal fixing property is generally used. When dealing with toner of heat fixing type, a temperature suitable for optimally forming an image before fixing is necessary and acceptable temperature range is generally narrow. Therefore, when the temperature of the toner is increased by the influence of frictional heat of the toner itself and a temperature rise of the parts in the apparatus, a print may be degraded due to a low charge amount of the toner and a low image density.
For this kind of thermal management, U.S. Patent Application Publication No. 2012/0301159 A1 discloses that a toner temperature sensor is set up in the apparatus and during image formation, image density and so on is controlled by adjusting the charge amount of the toner depending on a detected temperature rise of the toner.
However, in recent years, downsizing and cost reduction of image forming apparatuses are progressing and it has been difficult to provide a toner temperature sensor for directly detecting the temperature of the toner in view of space and costs. Therefore, in recent years, as described in Japanese Patent Laid-Open No. 2007-322539, a temperature detecting sensor is disposed at a place to which the temperature of toner is indirectly transmitted and the temperature of the toner is predicted by using a prediction equation based on the temperature of the place.
However, when placing a toner temperature sensor away from the toner as described in Japanese Patent Laid-Open No. 2007-322539, it takes time for the heat of the toner to transmit and reach the sensor. As a result, a temperature change detected by the sensor follows an actual temperature change of the toner with delay.
For example, in the case where a temperature difference between the inside of the apparatus and the outside air due to a temperature rise of the toner inside the image forming apparatus during a long time continuous printing, when air flow rate of a cooling fan is increased in order to cool the toner, flowing of the outside air of relatively low temperature into the apparatus increases. Then, a temperature change of the toner becomes gentle. At this time, the toner temperature detecting sensor follows an actual temperature change of the toner with delay. Thus, an error occurs between the temperature predicted from the detection results of the sensor and the actual temperature.
When the error occurs, the controlling in accordance with the image temperature as described in U.S. Patent Application Publication No. 2012/0301159 A1 is difficult. For example, a large image density change occurs on a replication print of the same original when a flow rate of the cooling fan changes. Because of this, a quality difference occurs among the prints which are printed at the same time or productivity of prints is lowered by adjusting images in order to prevent the quality difference. Thus, in the prior art, there is a problem that maintaining both quality and productivity is difficult.
When this quality difference occurs in prints of the same original, a user may easily recognize it even if the difference is slight. However, when a large air flow rate is set in advance to prevent the quality difference, anticipating a large safety factor, operating noise and power consumption are increased. Further, in this case, switching an air flow rate with the provision of a temperature detecting sensor is meaningless.